The applicant's British Pat. No. 1,361,436 describes a polyphase transformer of this kind, the so-called sector type, in which each magnetic core is obtained from a magnetic sheet which is wound on a mandrel of the same shape as the magnetic circuit, and is then annealed thereon. The core thus obtained is then cut up into units, which are threaded through both the coils for which they provide the magnetic circuit and they are then reassembled. Such transformers are satisfactory for moderate powers, but for the construction of high power transformers, e.g. above 315 KVA, they take a very long time to manufacture and it becomes excessively difficult to compact the laminations of magnetic sheets.
Indeed manufacture becomes very lengthy and the difficulties in compacting the magnetic sheets become excessive as soon as the thickness of the magnetic circuit increases somewhat. Further, for high levels of power, they do not stand up sufficiently to the electrodynamic forces acting on the magnetic circuits or on the coils. Because of their rectangular shape, the coils are difficult to manufacture with conductors of large cross-section, and they cannot withstand a complete short-circuit. Further still, it becomes impossible to maintain the levels of heating and noise to acceptable values without prohibitive increases in the quantities of iron in the magnetic circuit and of metal of high conductivity (copper or aluminium) in the coils.
Preferred embodiments of the present invention remedy the above drawbacks and enable manufacture of polyphase sector transformers even for high power levels said transformers being of relatively easy manufacture and sufficiently robust to withstand the electrodynamic forces adequately, even in the event of a complete short circuit, and having acceptable noise and heating levels, without using excessive quantities of iron and metal of high conductivity.